It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk deactivates the marker by using a deactivation device provided to deactivate the marker.
Known deactivation devices include one or more coils that are energizable to generate a magnetic field of sufficient amplitude to render the marker inactive. One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency and is generated by detecting apparatus, and the resonance of the marker is detected by the detecting apparatus. Typically, magnetomechanical markers are deactivated by exposing the bias element to an alternating magnetic field of sufficient magnitude to degauss the bias element. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
One deactivator device commercially provided by the assignee hereof employs a housing having an open side with a plastic bucket inserted in the housing such that an article or a plurality of articles may be placed in the bucket. Three coil pairs are disposed about the bucket in respective x-, y- and z-axis planes, whereby a strong demagnetization field is generated inside the bucket in each of the three orientations. In this device, the deactivation field is generated in the form of a pulse generated in response to the checkout clerk actuating a switch. Because of the three orthogonal coils provided in this device, effective deactivation occurs regardless of the orientation of the marker.
The assignee hereof commercially provides a second deactivation device that is manufactured at a lower cost than the first device and is easier to operate in connection with relatively large articles of merchandise. The second type of deactivator, sometimes referred to as a "pad" deactivator, employs one planar coil disposed horizontally within a housing. Articles of merchandise bearing markers are moved across the horizontal top surface of the housing. The pad deactivator includes detection circuitry with operates continuously or virtually continuously to detect the presence of markers and to briefly energize the deactivation coil on occasions when a marker is detected. A deactivator of this type is disclosed in U.S. Pat. No. 5,341,125.
FIG. 1 shows, somewhat schematically, a plan view of a deactivation coil of the type used in a typical commercial embodiment of a pad deactivator. The coil 12 shown in FIG. 1 is in the form of a 4-inch square. A marker to be deactivated is swept horizontally above the coil 12. Detecting circuitry (not shown) detects the presence of the marker, and triggers drive circuitry (also not shown) which temporarily energizes the coil 12 with an alternating current to form a deactivation field. The marker must be swept over the coil slowly enough so that the marker is detected and the coil energized before the marker leaves the vicinity of the coil.
A difficulty encountered with the coil arrangement shown in FIG. 1 is the variation in the effective peak demagnetization field amplitude experienced by the marker to be deactivated, depending upon the orientation of the marker as it is swept over the deactivation coil 12. The coil 12 provides the strongest magnetic field in the Z direction, which is the direction orthogonal to the plane of the coil 12. The magnitudes of the peak fields in the X and Y directions (parallel to the plane of the coil as indicated in FIG. 1) are substantially lower. FIG. 2 illustrates peak magnetic fields generated by the coil of FIG. 1, as a function of distance above the coil, when the coil is excited at a level of about 15,200 Amp-Turns (A-T) Curve 14 represents the Z direction field, as it varies with distance above the coil, while curve 16 indicates the lateral direction (X or Y direction) peak field, as it varies with distance above the coil.
It can be seen from FIG. 2 that the peak magnetic field in the Z direction is substantially greater than the lateral direction field at points 1 cm or more above the coil.
In one conventional variety of magnetomechanical EAS marker, the biasing element is formed as a 12.5 mm wide strip of a semi-hard magnetic material designated as "SemiVac 90", available from Vacuumschmelze, Hanau, Germany. When the length of the marker is aligned with the direction of the magnetic field, a peak field level of about 100 Oe suffices to degauss the biasing element enough to deactivate the marker. However, if the length of the marker is transverse to the field direction, a peak field level of about 200 to 300 Oe is required to deactivate the marker due to the increased demagnetization factor which occurs in this situation.
Referring again to FIG. 1, the coil 12 has branches 18 and 20 running in the Y direction and branches 22 and 24 running in the X direction. Current passing through the Y-direction branches 18 and 20 generates magnetic field components in the Z and X directions; similarly, current passing through the X-direction branches 22 and 24 generates magnetic field components in the Z and Y-directions. If a marker is oriented with its length parallel to the Y direction and is swept over the coil 12 along the locus indicated by the X axis in FIG. 1, then the dominant magnetic field components applied to the marker are substantially transverse to the marker length. Such is also the case with respect to a marker oriented with its length in the X direction and swept along the Y-axis locus. Since a 300 Oe transverse field is required for reliable deactivation, FIG. 2 indicates that the marker should not be swept at more than about 10 cm above the coil if deactivation is to be assured. It will be noted that at 10 cm there is a peak Z direction field of about 300 Oe, which would be transverse to a horizontally oriented marker.
Another conventional marker is only about 6 mm wide, and would require a field strength of about 600 Oe for reliable deactivation by a transverse field.
Furthermore, because of the high field level required for reliable deactivation, it is not feasible to continuously energize the deactivation coil, so that the prior art devices, as indicated before, are operated to generate the deactivation field only in occasional, short pulses initiated by user input or upon detection of a marker.
The difficulties in assuring that a sufficiently strong deactivation field is applied to the marker are exacerbated by the increasingly popular practice of "source tagging," i.e., securing EAS markers to goods during manufacture or during packaging of the goods at a manufacturing plant or distribution facility. In some cases, the markers may be secured to locations on the articles of merchandise which make it difficult or impossible to bring the marker into close proximity with conventional deactivation devices.